Will applying Decimate Modifiers speed up Blender?
Will it speed anything up if I apply all these decimate modifiers?

yes (you can try by yourself btw).
I suspect you are not using correctly blender. If you have a lot of faces, that means that the mesh comes from a sculpt operation or photogrammetry, so you DON'T have to edit the mesh in edit mode. If you stay away from edit mode, you should be able to work on your file.